Off-line Switcher. TNY254PN Datasheet
Energy Efficient, Low Power Off-line Switchers
Lowest Cost, Low Power Switcher Solution
• Lower cost than RCC, discrete PWM and other integrat-
• Cost effective replacement for bulky linear adapters
• Lowest component count
• Simple ON/OFF control – no loop compensation devices
• No bias winding – simpler, lower cost transformer
• Allows simple RC type EMI filter for up to 2 W from
universal input or 4 W from 115 VAC input
Extremely Energy Efficient
• Consumes only 30/60 mW at 115/230 VAC with no load
• Meets Blue Angel, Energy Star, Energy 2000 and 200 mW
European cell phone requirements for standby
• Saves $1 to $4 per year in energy costs (at $0.12/kWHr)
compared to bulky linear adapters
• Ideal for cellular phone chargers, standby power supplies for
PC, TV and VCR, utility meters, and cordless phones.
High Performance at Low Cost
• High-voltage powered – ideal for charger applications
• Very high loop bandwidth provides excellent transient
response and fast turn on with practically no overshoot
• Current limit operation rejects line frequency ripple
• Glitch free output when input is removed
• Built-in current limit and thermal protection
• 44 kHz operation (TNY253/4) with snubber clamp
reduces EMI and video noise in TVs and VCRs
• Operates with optocoupler or bias winding feedback
The TinySwitch family uses a breakthrough design to provide
the lowest cost, high efficiency, off-line switcher solution in
the 0 to 10 W range. These devices integrate a 700 V power
MOSFET, oscillator, high-voltage switched current source,
current limit and thermal shutdown circuitry. They start-up
and run on power derived from the DRAIN voltage, eliminat-
ing the need for a transformer bias winding and the associated
circuitry. And yet, they consume only about 80 mW at no load,
from 265 VAC input. A simple ON/OFF control scheme also
eliminates the need for loop compensation.
Figure 1. Typical Standby Application.
TinySwitch Selection Guide
for Lowest System Cost*
230 VAC or
4-10 W 3.5-6.5 W
Table 1. *Please refer to the Key Application Considerations section
The TNY253 and TNY254 switch at 44 kHz to minimize EMI
and to allow a simple snubber clamp to limit DRAIN spike
voltage. At the same time, they allow use of low cost EE16 core
transformers to deliver up to 5 W. The TNY253 is identical to
TNY254 except for its lower current limit, which reduces output
short-circuit current for applications under 2.5 W. TNY255
uses higher switching rate of 130 kHz to deliver up to 10 W
from the same low cost EE16 core for applications such as PC
standby supply. An EE13 or EF13 core with safety spaced
bobbin can be used for applications under 2.5 W. Absence of
a bias winding eliminates the need for taping/margins in most
applications, when triple insulated wire is used for the secondary.
This simplifies the transformer construction and reduces cost.
1.5 V + VTH
Figure 2. Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating
current for both start-up and steady-state operation.
BYPASS (BP) Pin:
Connection point for an external bypass capacitor for the inter-
nally generated 5.8 V supply. Bypass pin is not intended for
sourcing supply current to external circuitry.
ENABLE (EN) Pin:
The power MOSFET switching can be terminated by pulling
this pin low. The I-V characteristic of this pin is equivalent to
a voltage source of approximately 1.5 V with a source current
clamp of 50 µA.
SOURCE (S) Pin:
Power MOSFET source connection. Primary return.
TinySwitch Functional Description
TinySwitch is intended for low power off-line applications. It
combines a high-voltage power MOSFET switch with a power
supply controller in one device. Unlike a conventional PWM
(Pulse Width Modulator) controller, the TinySwitch uses a
simple ON/OFF control to regulate the output voltage.
The TinySwitch controller consists of an Oscillator, Enable
(Sense and Logic) circuit, 5.8 V Regulator, Undervoltage circuit,
P Package (DIP-8)
G Package (SMD-8)
Figure 3. Pin Configuration.
Hysteretic Over Temperature Protection, Current Limit circuit,
Leading Edge Blanking, and a 700 V power MOSFET. Figure
2 shows a functional block diagram with the most important
The oscillator frequency is internally set at 44 kHz (130 kHz
for the TNY255). The two signals of interest are the Maxi-
mum Duty Cycle signal (DMAX) which runs at typically 67%
duty cycle and the Clock signal that indicates the beginning of
each cycle. When cycles are skipped (see below), the oscilla-
tor frequency doubles (except for TNY255 which remains at
130 kHz). This increases the sampling rate at the ENABLE
pin for faster loop response.
Enable (Sense and Logic)
The ENABLE pin circuit has a source follower input stage set
at 1.5 V. The input current is clamped by a current source set
at 50 µA with 10 µA hysteresis. The output of the enable sense
circuit is sampled at the rising edge of the oscillator Clock
signal (at the beginning of each cycle). If it is high, then the
power MOSFET is turned on (enabled) for that cycle, otherwise
the power MOSFET remains in the off state (cycle skipped).
Since the sampling is done only once at the beginning of each
cycle, any subsequent changes at the ENABLE pin during the
cycle are ignored.
5.8 V Regulator
The 5.8 V regulator charges the bypass capacitor connected to
the BYPASS pin to 5.8 V by drawing a current from the voltage
on the DRAIN, whenever the MOSFET is off. The BYPASS pin
is the internal supply voltage node for the TinySwitch. When
the MOSFET is on, the TinySwitch runs off of the energy stored
in the bypass capacitor. Extremely low power consumption of
the internal circuitry allows the TinySwitch to operate continu-
ously from the current drawn from the DRAIN pin. A bypass
capacitor value of 0.1 µF is sufficient for both high frequency
de-coupling and energy storage.
The undervoltage circuitry disables the power MOSFET when
the BYPASS pin voltage drops below 5.1 V. Once the BYPASS
pin voltage drops below 5.1 V, it has to rise back to 5.8 V to
enable (turn-on) the power MOSFET.
Hysteretic Over Temperature Protection
The thermal shutdown circuitry senses the die junction tem-
perature. The threshold is set at 135 °C with 70 °C hysteresis.
When the junction temperature rises above this threshold
(135 °C) the power MOSFET is disabled and remains disabled
until the die junction temperature falls by 70 °C, at which point
it is re-enabled.
The current limit circuit senses the current in the power
MOSFET. When this current exceeds the internal threshold
(ILIMIT), the power MOSFET is turned off for the remainder of
The leading edge blanking circuit inhibits the current limit
comparator for a short time (tLEB) after the power MOSFET
is turned on. This leading edge blanking time has been set so
that current spikes caused by primary-side capacitance and
secondary-side rectifier reverse recovery time will not cause
premature termination of the switching pulse.
TinySwitch is intended to operate in the current limit mode.
When enabled, the oscillator turns the power MOSFET on at the
beginning of each cycle. The MOSFET is turned off when the
current ramps up to the current limit. The maximum on-time
of the MOSFET is limited to DCMAX by the oscillator. Since
the current limit and frequency of a given TinySwitch device
are constant, the power delivered is proportional to the primary
inductance of the transformer and is relatively independent of
the input voltage. Therefore, the design of the power supply
involves calculating the primary inductance of the transformer
for the maximum power required. As long as the TinySwitch
device chosen is rated for the power level at the lowest input
voltage, the calculated inductance will ramp up the current to
the current limit before the DCMAX limit is reached.
The TinySwitch senses the ENABLE pin to determine whether
or not to proceed with the next switch cycle as described earlier.
Once a cycle is started TinySwitch always completes the cycle
(even when the ENABLE pin changes state half way through the
cycle). This operation results in a power supply whose output
voltage ripple is determined by the output capacitor, amount of
energy per switch cycle and the delay of the ENABLE feedback.
The ENABLE signal is generated on the secondary by comparing
the power supply output voltage with a reference voltage. The
ENABLE signal is high when the power supply output voltage
is less than the reference voltage.
In a typical implementation, the ENABLE pin is driven by
an optocoupler. The collector of the optocoupler transistor is
connected to the ENABLE pin and the emitter is connected to
the SOURCE pin. The optocoupler LED is connected in series
with a Zener across the DC output voltage to be regulated.
When the output voltage exceeds the target regulation voltage
level (optocoupler diode voltage drop plus Zener voltage), the
optocoupler diode will start to conduct, pulling the ENABLE
pin low. The Zener could be replaced by a TL431 device for
The ENABLE pin pull-down current threshold is nominally
50 µA, but is set to 40 µA the instant the threshold is exceeded.
This is reset to 50 µA when the ENABLE pull-down current
drops below the current threshold of 40 µA.
The internal clock of the TinySwitch runs all the time. At the
beginning of each clock cycle the TinySwitch samples the
ENABLE pin to decide whether or not to implement a switch
cycle. If the ENABLE pin is high (< 40 µA), then a switching
cycle takes place. If the ENABLE pin is low (greater than
50 µA) then no switching cycle occurs, and the ENABLE pin
status is sampled again at the start of the subsequent clock cycle.
At full load TinySwitch will conduct during the majority of
its clock cycles (Figure 4). At loads less than full load, the
TinySwitch will “skip” more cycles in order to maintain volt-
age regulation at the secondary output (Figure 5). At light
load or no load, almost all cycles will be skipped (Figure 6).
A small percentage of cycles will conduct to support the power
consumption of the power supply.